Freezing a gas component in a gas mixture

ABSTRACT

The invention relates to a method and a device for separating a gas component in a gas mixture by freezing. The gas component to be separated has the lower vapor pressure when compared with the other gas components in the gas mixture. A working temperature is chosen at which the gas components to be separated freezes out almost completely. The partial pressure of the other gas components at working temperature is kept lower than the vapor temperature of the other gas components at working temperature. Once the components to be separated have been frozen out, the other gases are evacuated. The device comprises at least two cooling tanks. Means are provided to alternately feed the gas mixture to the cooling tanks so that a predetermined partial pressure arises in each tank.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national stage of PCT/DE 98/03298 filed Nov. 5,1998 and based upon German national application 19749836.1 filed Nov.11, 1997 under the International Convention.

FIELD OF THE INVENTION

The invention relates to a process for separating a gas component from agas mixture by freezing out.

BACKGROUND OF THE INVENTION

From the publications DE 35 18 283 C2 and DE 196 35 002 A1, a process isknown in which a gas to be continuously separated is evacuated from acold trap. In this manner high degrees of purity are obtained.

A process of this type is also known from the publication DE 34 22 417A1 for separating a gas component from a gas mixture by freezing out. Acold trap with a freezing stretch is provided. In the cold trap, the gasis subjected over a long residence time and on a very large cooling areato intensive heat exchange and mass exchange with the cooling orcondensing surfaces. The cooling surfaces are generally formed as sheetsrunning transversely to the gas flow direction.

A high degree of supersaturation of the gas within the cooling trap canthus be reliably avoided. The cold trap has no constrictions tendingtoward plugging up. The temperature is so controlled and monitored overthe freezing stretch that at the gas input of the latter, the saturationtemperature of the component to be frozen out prevails and at the outletthe saturation temperature corresponding to its desired residualconcentration is maintained. Between the two, efforts are made toprovide a substantially linear temperature profile. For that purpose,temperature sensors on the cooling sheets and a stepwise control of thestretch traversed by the coolant flow and additional heating withintervening heat damming stages are provided.

The known process requires a relatively high cost apparatus.

OBJECTS OF THE INVENTION

It is the object of the invention to provide a simple process of thetype described in which the component to be separated can be obtainedcompletely and with high purity. It is also an object of the inventionto provide a process which can be carried out in a quasi continuousmanner.

SUMMARY OF THE INVENTION

The objects are achieved with a process for separating out a gascomponent from a gas mixture by freezing out with the features:

a) the gas component to be separated has by comparison to the remaininggas components in the gas mixture, the smallest vapor pressure,

b) an operating temperature is selected at which the gas component to beseparated is frozen out,

c) the gas mixture is subjected to the operating temperature,

d) the partial pressures of the remaining gas components at theprevalent working temperature are held smaller than their vaporpressures at the operating temperature, and

e) after the freezing out of the component to be separate, the solidleft behind is separated from the remaining gases.

The remaining gases are separated by pumping them off from the solidleft behind. The frozen gas separated from the remaining gases is thenheated. An apparatus for carrying out the process can have

a) at least two cooling vessels which are cooled to the operatingtemperature by coolant are provided,

b) each of the two cooling vessels has a closeable inlet and outlet,

c) a pump is connected with each of both outlets,

d) means are provided which alternately feed the gas mixture to one andthe other cooling vessel so that in the respective vessel apredetermined partial pressure is established.

The apparatus can have cooling sheets of increased surface area in thevessels.

According to the invention gas components are separated from a gasmixture which have the smallest vapor pressure. The vapor pressure ofthe remaining (other) components in the gas mixture should be clearlygreater in comparison to the aforementioned smallest vapor pressure. Theprocess can be carried out especially well and economically.

A working temperature should be so selected that the vapor pressure ofthe gas component to be separated is practically zero. The separation ofthe gas component with the smallest vapor pressure can then be effectedespecially economically.

A higher operating cost is not required.

An example of a gas mixture according to the claimed invention is amixture of xenon and oxygen with 70 volume % xenon and 30 volume %oxygen. At a working temperature of 78 K., the vapor pressure of the gascomponent xenon is about 10⁻³ mbar and is approximately zero in theaforementioned sense. The vapor pressure of the gas component oxygenamounts to 228 mbar at 78 K and thus is significantly greater than thatof xenon in the aforementioned sense.

According to the invention, the gas mixture is subjected to the workingtemperature. The prevailing partial pressure at the working temperature(or the partial pressures which predominates when three or more gascomponents are present in the gas mixture) of the gas component which isnot to be frozen out should be less than the vapor pressure (prevailingat this working temperature) of this gas component. This insures thatthese gas components will not be condensed out or frozen out. In thecase of xenon-oxygen gas mixtures the oxygen partial pressure of theoxygen component should be held less than 228 mbar when the gas mixtureaccording to the invention is at its working temperature of 78 K. Anoperating temperature of 78 K can be economically achieved by coolingwith liquid nitrogen. The oxygen partial pressure especially should beheld at less than 200 mbar during the freezing out to obtain the desiredresults.

The gas component to be separated is thus frozen out. The other gascomponent or other gas components remain gaseous. If the latticecharacteristics of the gas (in the frozen state) are sufficientlydifferent, molecules or atoms of the other gas components will usuallynot be trapped in the lattice of the resulting solid body.

Several gas molecules or atoms are at most bound by adsorption to thesurface of a resulting solid body.

A sufficient difference between the respective lattices is present, forexample, between frozen xenon and frozen argon or between frozen xenonand frozen oxygen.

Since the gas components with the smallest vapor pressure are frozen outat the prevalent working temperature, the other gas components can beseparated for example by pumping them off from the frozen gas. Withincreasing pumping duration, the adsorptively bound impurities arereduced by desorption and thus the degree of purity is increased.Following subsequent warming of the frozen gas there is produced bysublimation a gas with a correspondingly high purity.

In the case of xenon (90 volume %) argon (10 volume %) gas mixtures,after the freezing out it is only necessary to apply a prevacuum pump(two-stage rotary disk pump) for several minutes to obtain xenon with apurity of 99.9%.

The process is especially useful with xenon-nitrogen gas mixtures andxenon-krypton-nitrogen gas mixtures for the separation of xenon.

The invention can be carried out with very simple means and thus veryinexpensively.

An apparatus for carrying out the process includes at least two coolingvessels (cooling traps, freezing stretches) which are cooled to workingtemperature. Each cooling vessel has a closeable inlet and outlet. Apump is connected with each outlet. The device further comprises meansfor alternately supplying the gas mixture to one and the other coolingvessel so that in the respective cooling vessel the desired partialpressure will be established. By the alternate supply of the gas mixturea continuous operation can be insured. If in one of the vessels there isa gas mixture or a gas mixture is passed through this vessel (flowthrough operation), the component to be separated is frozen out and theremaining gas is pumped out. Only the frozen out gas (solid) thusremains in the vessel. The solid can then be removed from thecorresponding vessel in a suitable way. This can be achieved for exampleby heating. The resulting over pressure can be directly used to fill agas bottle under pressure.

A 100% recovery of, for example, xenon from a gas mixture is thuspossible without problems.

The process can basically be used with any gas mixture which fulfillsthe aforementioned conditions.

The apparatus has, in an advantageous embodiment, means for admittingthe gas mixture into a vessel within which the gas mixture is conductedover cooled surfaces (cooling surfaces). This can insure that the gasmixture in every case will be guided over cooled surfaces on which thegas can be frozen out. The freezing out can be economically and reliablyeffected in a flow through operation of the gas mixture through thevessel.

In a further advantageous configuration, the apparatus can have surfaceenlarging cooling plates in the cooling vessel which can therebyincrease the cooling power.

What is claimed is:
 1. A process for separating a gas component from agas mixture containing at least two gas components comprising the stepsof: a) providing an operating temperature at which the gas component tobe separated has by comparison to any remaining gas component in the gasmixture, the smallest vapor pressure and at which the gas component tobe separated is frozen out; b) subjecting the gas mixture to theoperating temperature while maintaining partial pressures of gascomponents remaining in a gaseous state at said operating temperaturesmaller than respective vapor pressures of said gas components remainingin said gaseous state at the operating temperature, thereby freezing outthe gas component to be separated in a solid; and c) after freezing outof the component to be separated, separating said solid from a remaininggas.
 2. The process according to claim 1 in which the remaining gas isseparated from the solid by pumping it off from the solid left behind.3. The process according to claim 2 in which the solid after separationfrom the remaining gas is then heated to form a gas phase of the gascomponent to be separated.